With aging there is an increase in fracture risk, due largely to a progressive decline in bone mass and strength (osteoporosis). An estimated 2 million osteoporotic fractures occur annually in the U.S. at a cost of $17 billion. The predominant treatment for osteoporosis is the use of drugs that block bone resorption (anti-resorptive or anti-catabolic). Yet, a main feature of skeletal aging is a decline in the rate of bone formation, and there exists an unmet clinical need for anabolic strategies to offset this decline. When mechanical forces are applied to bones cyclically (i.e., load-unload, load-unload, etc.) they can stimulate an increase in bone formation that leads to accrual of bone mass. Thus, mechanical loading of the skeleton represents a powerful anabolic strategy with potential to treat osteoporosis. However, it is unclear if the aging skeleton loses its ability to respond to loading. The long-term goal of this project it to determine how age influences the biological response of bones to mechanical loading, i.e., mechanoresponsiveness. Results from the past funding period indicate that aged mice responded to loading as well as younger mice, which leads to the overall hypothesis - age does not limit the ability of mechanical loading to stimulate increased bone formation. In order for bones from aged animals to increase bone formation, they must overcome a low baseline rate of bone formation and a lack of committed osteoblasts. Thus, loading must first recruit osteoblasts at the site of bone formation by differentiation of uncommitted lining cells or osteoprogenitors, a process that may also involve cell proliferation. One mechanism by which bone formation may be stimulated at all ages is through Wnt/Lrp signaling. Using the tibial compression loading model, we propose to determine the relative importance of osteoblast recruitment and the requirement for Wnt/Lrp signaling in the mechanoresponse of bones from mice across their life spans. Aim 1A: Apply in vivo mechanical loading to mice at multiple ages, and examine responses to loading at the tissue level (microCT, dynamic histomorphometry) and the molecular level (qPCR gene expression). Aim 1B: Using an osteoblast reporter mouse, determine the effect of mechanical loading on local osteoblast recruitment as a function of age. Aim 1C: Using mice in which replicating osteoblast progenitors are conditionally ablated, determine the importance of cell proliferation on loading-induced bone formation as a function of age. Aim 2A: Using a Wnt reporter mouse, evaluate the relationship between mechanical strain magnitude and activation of canonical Wnt signaling in bones from young and old mice. Aim 2B: Determine if interruption of anabolic Wnt/Lrp signaling in osteoblasts precludes the anabolic response of bones to mechanical loading in young and old mice. Our findings will contribute to the field of skeletal biology in two important ways: 1) they will either support or refute the potential of loading to increase bone mass in the aged skeleton, and 2) they will identify age-related differences (and similarities) in cellular and molecular responses to loading.